1. Field of the Invention
The field of this invention is generally target fuzing and specifically air-target fuzing, although many types of surface targets can be served, too. The invention also relates to the fields of: (1) Air-Targets-Aircraft, Helos, Missiles, RV's and RPV's; (2) WideAngle, Body-Fixed, and Passive Imaging-infrared Sensing and Target Detection Devices; (3) Skewed-Cone Fuzing with Aim-Point Selection and Directional-Warhead aiming; and (4) Non-Spinning or Slowly-Spinning weapons.
2. Prior Art
The first anti-aircraft projectile proximity fuze, a radio-frequency-field motion detector. was developed in 1942. It provided very crude target location, literally proximity, based on signal amplitude, and detonated a nearly-omni-directional blast-fragment warhead.
Near the end of World War II, evolving radar and anti-aircraft missile technology combined to produce the fixed-angle microwave fuze. Centimeter-wavelength antennae about the missile periphery created a forward-looking right cone of revolution at a fixed angle about the longitudinal axis. Radar echoes from targets crossing the conical beam detonated a fragmenting warhead somewhat focused normal to the missile's longitudinal axis with an approximate lead time. This degree of target location enabled a few hundred pounds of explosive to kill relatively-slow aircraft at tens of feet.
The hollow-cone sensor actually served two functions. It located the target with fair precision in missile/warhead space, and its shape provided an elegant fire control algorithm. Given a planar fragmenting warhead, i.e., one focused into a tight circumferential spray roughly normal to the missile's longitudinal axis, upon detonation., an expanding ring of fragments flew outward with a velocity V.sub.w, and forward with a velocity V.sub.M, forming a right cone about the missile's longitudinal axis having a half-angle of .notident.=tan.sup.-1 V.sub.w /V.sub.M, the so-called dynamic fragmentation pattern (see FIG. 1). Now if the fuzing cone were coincident with the dynamic fragment cone, the fire control algorithm was ultimately simple: fire when you saw the target, without regard for miss distance, and the target and fragments each would travel to assured intercept--a truly-elegant, angle-only solution.
Or almost a solution. In actuality, two major sources of fuzing error have been neglected above: (1) the effects of target motion, and (2) the location of the target's so-called center of vulnerability (COV) within the fuze detection surface (generally not the target's physical surface). Note that the target velocity could be taken into account completely if the fuzing cone were skewed by adding -V.sub.T, the negative target velocity vector, to V.sub.M, to form V.sub.R, the relative velocity vector (see FIG. 2). Unfortunately, this proved impractical to do with vacuum tubes and waveguide antennae. (Even variable-angle right cones were found impractical). And no solutions to the COV problem a crude radar or thermal centroiding beyond were offered with an approximate time delay between target detection and firing to permit COV approach.
Thus, the last 50 years of air-target fuzing have been one continuous attempt to compensate a fixed-angle fuze for these two sources of error. (Not to overlook progress in materiel from vacuum tubes to microchips, fuze beam resolution using shorter wavelengths, signaling techniques for improved clutter and counter-measure resistance, etc.). This attempt has centered on tilting the sensing cone forward to permit the use of a fixed time delay based on various built-in estimates or, later, a variable time delay based on fuze, guidance, or fire control observables (target-type, velocity, and heading; long-range line-of-sight (LRLOS), miss distance, miss quadrant, etc.), and adding/dividing sensing cones for additional position/extent fixes.
However, during the last 50 years missile and target speeds have increased, generally more than fragment speeds, so fuzing cones have been pushed forward. At best this means larger time delays to cover all encounters, and larger fuzing errors with incomplete/inaccurate encounter information. Warhead beamwidths have been increased to fill the larger volumes of uncertainty with a consequent reduction in lethality. At worst, it means that in high-speed crossing shots, targets can slide in behind a single fuze cone without being seen! For the next generation of targets, such as RV's, the system of fixed-angle fuzing of planar-fragmenting warheads threatens to break down completely, as in Patriot.
As evidence of the state of the art as reflected in issued U.S. patents, a search of the fuze and related classes has disclosed 10 patents, listed below numerically:
U.S. Pat. No. 3,046,892 Cosse et al--Spinning Projectile, Optical/Radio Fixed-Angle, Non-lmaging/Non-Centroiding, Planar Warhead. PA1 U.S. Pat. No. 3,242,339 Lee--Non-Spinning, Multiple-Optical Fixed-Angle, Non-lmaging/Non-Centroiding, (Directional Warhead). PA1 U.S. Pat. No. 3,942,446 Cruzan--Non-Spinning, Multiple-Optical Fixed-Angle, Non-Imaging/Non-Centroiding, (Directional Warhead). PA1 U.S. Pat. No. 4,168,663 Kohler--Non-Spinning, Radar Time-To-Go, Non-lmaging/Non-Centroiding, Planar Warhead. PA1 U.S. Pat. No. 4,203,366 Wilkes--Non-Spinning, Radar Fixed-Angle, Non-lmaging/Non-Centroiding, Planar Warhead. PA1 U.S. Pat. No. 4,599,616 Barbella et al--Non-Spinning, Radar Fixed-Angle, Non-lmaging/Non-Centroiding, Planar Warhead. PA1 U.S. Pat. No. 4,625,647 Laures--Non-Spinning, Radar Fixed-Angle, Non-imaging but Centroiding, Planar Warhead. PA1 U.S. Pat. No. 4,627,351 Thorsdarson et al--Spinning Projectile, Optical/Radar Fixed-Angle, Non-lmaging/Non-Centroiding, Directional Warhead. PA1 U.S. Pat. No. 4,630,050 Johnson--Non-Spinning, Radar Time-To-Go Seeker-Fuze, Non-lmaging/Non-Centroiding, Planar Warhead. PA1 U.S. Pat. No. 4,895,075 Munzel--Spinning Projectile, Wide-Angle-Radar Skewed-Cone (Approx.) Non-lmaging/Non-Centroiding, Planar Warhead.
Not one disclosure uses imaging, with all its benefits, including aim-point selection, and only one disclosure (Laures) attempts aim-point selection (by delaying firing after presumed nose (or tail) detection). Only one disclosure (MCinzel) approximates skewed-cone fuzing by happenstance--with a right cone centered on V.sub.R and only three disclosures permit or use a directional warhead.
Analyzing the shortcomings of Fixed-Angle Fuzing to guide the next generation system design, the following is needed: (1) A skewed-cone firing algorithm which conforms to actual (or estimated) target vector velocity, both to increase fuzing accuracy and to preclude blind crossing shots; (2) Target feature recognition to permit aim-point selection and highest kill probability; and (3) A prediction of miss direction to permit high-lethality directional warheads. These features will enable an aimable or steerable warhead to be aimed/steered in the predicted miss direction of the desired target aim point and detonated when said aim point crosses the skewed fuzing cone, resulting in a so-called optimum burst point from which the most fragments will impact the (presumed) most vulnerable portion of the target. One way of satisfying these requirements would be to provide an annular optical sensor or array of sensors projecting the skewed-cone onto a movable ring of detectors as in FIG. 3. The problems of this approach are expense, fragility, and crude, partial and late imaging as the target sweeps past the skewed-cone, requiring imprecise aim-point selection and millisecond warhead aiming.
The preferred way of fulfilling all three of these requirements is by a wide-angle, imaging-infrared sensor in which the skewed-cone algorithm, the aim-point selection, and the miss-direction prediction are accomplished in image-processing software.